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The invention pertains to the general field of scanning and in particular to two-dimensional scanning of flat objects.
Scanning is a central aspect of many industrial, commercial and technological processes. This is done either to collect information by numerous sensing means, such as in optical scanning where optical data is gathered over a relatively large area, or with and actuator head where the scanning is undertaken with a view to effect an influence on a point by point basis over the area being scanned. The present invention concerns itself with 2-dimensional scanning where there is no physical contact between the scanning device and the object being scanned.
Prior art in the field of has established the practice of ensuring correct two-dimensional positioning of scanning devices by driving the scanning system to predetermined positions based on incremented coordinates in both the X and Y directions. In some cases the object being scanned is placed on a platen and the latter is moved along a 2-dimensional raster path, while in others the scanning head is moved relative to the object to be scanned which is affixed to a stationary platen. Both situations have in common the fact that the control system assumes the coordinates of the position that is scanned or illuminated to be those to which the scanning head or platen has been directed by the control system.
U.S. Pat. No. 4,245,259 teaches an X-Y scanning device in which the coordinates of the scanned point are not predetermined, but instead either the scanning head or the scanned platen moves with respect to the other member in at least one direction in a free running oscillatory shuttle motion, the initial and make-up power being provided by a low inertial motor and the return stroke provided by energy conserving springs of suitable stiffness at the end of the travel of the scanning head. In this case the instantaneous position of the moving member is determined by measurement and is not predetermined through some programmed means. This particular invention seeks to attain a smooth and uniform scanning velocity utilizing a minimum of drive power. The invention incorporates a conventional roller drive.
U.S. Pat. No. 2,476,496 teaches the use of linear motors in scanning devices, the invention addressing itself in particular to a multipurpose facsimile machine. The driven member moves along a guide rail and terminates against shock absorbing springs. The linear motor in this invention comprises coils in the moving member and alternating magnetic and non-magnetic materials in the guide rail along which the moving member glides with the coils concentric to the guide rails.
U.S. Pat. No. 4,348,697 describes a facsimile apparatus a similar machine which generates signals representing information for more than one scan line during each traversal of the scanning head or carriage.
U.S. Pat. No. 4,764,815 discloses yet another shuttling scanner. This particular invention addresses itself, amongst other aims, to the reduction of the mechanical forces involved in larger and heavier high resolution scanning devices. It seeks to do so by means of springs, either mechanical or air, at either end of the shuttle traverse. In this invention the springs are supported independently from either the scanning head or the platen being scanned.
U.S. Pat. No. 4,750,721 seeks to address the forces that destabilize scanning devices by using a counter weighting approach that seeks to maintain the center of mass of the system as a whole in a fixed position. It does so by means of an intermediate stage operating much like a recoilless system. It also employs a linear motor as drive system.
The above prior art therefore clearly discloses the concept of a scanning device in which at least one of either the platen or the scanning head moves in at least one dimension by a ballistic shuttling motion, the position of the moving part being determined by measurement rather than by driving it to a predetermined position. Means of reducing the mechanical forces on the scanner as a whole, with a view to attaining positioning accuracy, are also disclosed in this prior art.
Within the semiconductor integrated circuit manufacturing industry there are applications requiring extremely accurate positioning of scanning heads and platens. Often these systems are comparatively large, representing significant mass and momentum in their moving parts. Simultaneously this industry has great concern regarding the generation of particulate matter that go directly to the quality of the product being produced. This results in the equipment being placed in Class 10 clean facilities with extreme precautions regarding the generation and disturbance of particulate matter. Any scanning devices generating significant forces whilst executing shuttling actions will therefore run the risk of freeing up such particulate matter and exacerbating the quality concerns. Any equipment containing rigid surfaces that come into contact periodically is undesirable, as such contact generates particulate matter. All prior art exhibits this problem.
There is therefore considerable merit in devising a means of reducing the peak forces required to drive the scanning system. There is also merit in ensuring that neither the traverse nor the execution of the return stroke of shuttling scanners requires physical contact between any bulky parts in mutual motion as such contact would similarly be a source of forces leading to loosened particulate matter and consequent contamination.
Within the printed circuit board industry there is an added consideration that the boards should be scanned on both sides simultaneously, thereby eliminating a costly additional processing step. There is therefore considerable merit in an approach that provides this additional functionality.
In the specific area of the reciprocation of the shuttling part of scanners, the prior art described in U.S. Pat. No. 2,476,496 teaches the use of springs while U.S. Pat. No. 4,764,815 describes the use of either springs or air springs. Both of these devices have shortcomings that lead to excessive forces being generated during the reciprocation process of the shuttling part. In the case of a spring the force in the opposite direction to the motion of the shuttle increases linearly with the compression while in the case of air springs it increases even more rapidly with compression. The result is quite a precipitous stop to the shuttle with resulting high peak forces. There is therefore much merit in devising an alternative means to effect the slowing and turnaround reciprocation of the shuttle carriage of the scanner if this means will lead to lower peak forces during the stroke of the shuttle carriage.
In accordance with the present invention a scanning method for flat plate objects obtains reciprocating ballistic motion without any physical contact between the mutually moving parts. The method ensures that the peak forces attained during scanning are minimized to prevent dislodgement of particulate material. The method furthermore ensures that both surfaces of flat plate objects may be scanned simultaneously.